This invention relates to a measurement method and apparatus therefor and, more particularly, to a method and apparatus for measuring the coordinates of a point on an object, or the distance to the object.
A method is available in which a measuring element is brought into contact with the surface of an object to measure the coordinates of a point on the object surface or the distance to the object. A method of measuring depth, for example, includes (a) commanding an amount of movement in excess of actual depth, (b) generating distributed pulses by performing a pulse distribution computation based on the commanded amount of movement, (c) transporting a measuring element by driving a servomotor in response to the distributed pulses, and (d) bringing the measuring element to rest after it contacts the object, and measuring the depth by relying upon the actual amount of movement the measuring element made before coming to rest.
The servo system which drives the servomotor controls the motor in such a manner that a difference (positional deviation) Er, between the number of distributed pulses and a number of pulses sensed each time the measuring element is moved by a prescribed amount, will approach zero. Specifically, assume that the distributed pulses ordinarily have a pulse rate of Fi, and that the gain of the servo system is k. In such case, the servomotor will be controlled so as to rotate with a delay equivalent to Er=Fi/k, under steady conditions, so that the delay will approach zero. The delay Fi/k (pulse number) is the difference between the number of distributed pulses and the number of sensed pulses, and is stored in an error counter. When the distributed pulses have ceased being generated, the servomotor will come to rest after rotating by an amount corresponding to the aforesaid difference (i.e., positional deviation).
The foregoing leads to a problem in the conventional measuring method, which will be understood from FIG. 1. As described above, the measuring element, represented by ME, contacts the surface of the object being measured, designated by MW. Even though the generation of distributed pulses is halted during such contact, however, the measuring element will continue to travel a distance equivalent to Fi/k (number of pulses), and will bite into the object as a result. This makes it impossible to obtain an accurate measurement of position. An attempt at a solution has been to minimize the distributed pulse rate (i.e., feed speed) Fi of the measuring element ME in order to reduce the deviation Fi/k. However, a disadvantage with this expedient is that the lower pulse rate prolongs the time needed for measurement.